Providing the RAM is DDR (not DDR2 or DDR3) then adding it will do no harm, and should help speed things up a little. It can go in with the existing RAM, which should remain in their existing slots.
I would think that the D950 / D960 CPUs would work, and again, will provide a bit of a performance boost (the D950/60 CPUs are dual core, rather than single core). A quick search on Google shopping showed a couple of second hand examples for not much money. My only question would be if the motherboard can support 800MHz front side bus (FSB) speed. Your existing CPU is 533MHz. If your motherboard can't support the higher FSB then the new CPU should still work, you just won't get the full speed benefit from it.
Noises from computers only usually come from two things: fans and hard drives. In your case, I'd guess the hard drive. It may well have many hundreds of hours of life left in it.....or it may not!